It's for updates to all MS products. Before you had to search for OS and Office products separately; now the updates page cehecks for both at the same time.
If you visit the link:
http://update.microsoft.com/microsoftupdate/v6/default.aspx
with IE, you will need to accept a couple of ActiveX installations to update update.
If you visit with Firefox, you won't see anything, of course.
If I use the Windows Update tool, either from Start or from the IE Tools menu, I get sent to v5 same as before, with a standard "old" scan".
The very first thing it says it's doing, as usual, is checking for latest version of Win Update software. But nothing comes of that except carrying on with the normal "main" scan.
Is v6 maybe still beta and not yet "official", despite announcements that seem to indicate otherwise?
(Several hours later) Using Frank's link, v6 installed just fine for me. I'm mildly surprised that IE's Tools menu entry (for Win update) hasn't been renamed, but that now takes me to the MS v6 page instead of the Win v5 page as before.